1. Field of the Invention
The present invention relates to an apparatus and method for Direct Current (DC) offset calibration in a system. More particularly, the present invention relates to an apparatus and method for addressing a DC offset problem on a transmission path caused by a frequency leakage of a local oscillator in a system using a direct-conversion modulator.
2. Description of the Related Art
Most wireless communication transmitters/receivers adopt and use an analog heterodyne transmission/reception scheme to perform up/down frequency conversion several times using several mixers and Intermediate Frequency (IF) ends for a transmit/receive signal. However, a conventional transmission/reception structure using a plurality of analog parts has a disadvantage in that its circuit is complex and is difficult to be integrated into one chip, thus causing an increased volume of a transmitter/receiver using the transmission/reception structure. Further, power consumption of the transmitter/receiver using the transmission/reception structure increases and thus, the transmission/reception structure is not suitable for products that consider miniaturization and low output as important, such as Picocells, Ubicells, and the like.
In order to address the above-identified problems, a transmission/reception structure using a Radio Frequency Integrated Circuit (RFIC) adopting a direct-conversion modulator as a basic structure is currently used. The transmission/reception structure will be more widely used in a transmitter/receiver of a miniaturized and low power system in the future. The transmission/reception structure adopting the direct-conversion modulator performs frequency up/down conversion one time using one mixer for a transmit/receive signal and thus, may reduce the number of various kinds of filters, amplifiers, and peripheral circuits that should be used whenever one mixer is used. Accordingly, the transmission/reception structure has a simple circuit and is easily integrated into one chip, thus decreasing cost of a transmitter/receiver using the transmission/reception structure.
However, the transmission/reception structure adopting the direct-conversion modulator causes many problems in performance. For instance, the transmission/reception structure causes a mixer related problem as well as an oscillation problem, a selectivity problem, and the like. More particularly, the transmission/reception structure causes a DC offset problem due to a frequency leakage of a local oscillator and fatal problems, such as noises, and the like, occurring near a DC band. Also, the transmission/reception structure causes an In-phase/Quadrature-phase (I/Q) imbalance problem due to a mismatch of phase and amplitude between an I channel and a Q channel.
Among the problems, the DC offset problem is a problem in which an undesired DC component is included in analog I and Q signals originally intended for transmission. The DC offset problem results from a leakage of a predefined amount of carrier frequency components irrelevant to a signal originally intended for transmission in a local oscillator and self-mixing of the carrier frequency components with the signal originally intended for transmission. If a DC component output value measured by a power meter at an antenna end of a system exceeds a reference value allowable in the system, the system may determine that the DC offset problem occurs.
The DC offset problem may make restoration of a receive signal impossible, and may greatly rotate the constellation of the receive signal, thus resulting in a deteriorating Bit Error Rate (BER) performance. Recent technology for canceling noises near a DC band has progressed due to developments of circuit and chip technology. However, the DC offset problem on the transmission path remains because most of the RFICs perform self DC offset calibration only from the standpoint of a reception side's performance.
Therefore a need exists for an apparatus and method for addressing a DC offset problem on a transmission path caused by a frequency leakage of a local oscillator in a system using a direct-conversion modulator.